As is well known, one source of noise in active pixel image sensors is thermally generated or kT/C noise, which is generated when the active pixels are released from reset. As pixels are decreased in size, kT/C noise becomes the dominant temporal noise source. A typical 5.6 μm pixel with 3fF capacitance will have kT/C noise of 1.67 mV rms.
European Patent Application 01310582.0 filed Dec. 19, 2001, by the present Assignee, discloses a “double read” system in which reset values for each line and signal values for each line are read out line-by-line and interleaved, with the reset values being held in a frame buffer for later subtraction from corresponding read values. The number of line periods separating the reset and read signals are selectable to control the image exposure. Although this prior approach is effective in reducing noise, it does require a relatively large frame buffer.